Method for separating a useful layer and component obtained by said method

ABSTRACT

A useful layer ( 1 ) is initially attached by a sacrificial layer ( 2 ) to a layer ( 3 ) forming a substrate. Before etching of the sacrificial layer ( 2 ), at least a part of the surface ( 4, 5 ) of at least one of the layers in contact with the sacrificial layer ( 2 ) is doped. After etching of the sacrificial layer ( 2 ), the surface ( 4, 5 ) is superficially etched so as to increase the roughness of its doped part. After doping, a mask ( 9 ) is deposited on a part of the useful layer ( 1 ) so as to delineate a doped zone and a non-doped zone of the surface ( 4, 5 ), one of the zones forming a stop after the superficial etching phase.

BACKGROUND OF THE INVENTION

The invention relates to a method for separating a useful layer,initially attached by a sacrificial layer to a layer forming asubstrate, method comprising

-   -   at least partial etching of the sacrificial layer,    -   doping, before etching of the sacrificial layer, of at least a        part of the surface of at least one of the layers in contact        with the sacrificial layer and,    -   after etching of the sacrificial layer, a superficial etching        phase of said surface so as to increase the roughness of the        doped part of the surface.

STATE OF THE ART

Certain micromechanical components, for example actuators oraccelerometers, comprise a suspended useful layer, attached by fixingmeans to a substrate. The distance between the useful layer and thesubstrate can be in the region of or less than one micron. In this case,the component is generally fabricated by means of a sacrificial layerenabling the distance between the useful layer and the substrate to bemonitored. As represented in FIG. 1, the useful layer 1 is initiallyattached by the sacrificial layer 2 to a layer forming a substrate 3.When fabrication takes place, the sacrificial layer 2 is at leastpartially etched to obtain a suspended structure.

Etching is typically performed by liquid chemical means, possiblyfollowed by rinsing. After etching and rinsing, the component is driedand capillary forces may attract the useful layer 1 towards thesubstrate 3 thus causing sticking of their opposite surfaces 4 and 5,which makes the component unusable. Other forces, for exampleelectrostatic forces or Van der Waals forces, may also lead to stickingof the surfaces 4 and 5.

In FIG. 2, sticking of the surfaces 4 and 5 is prevented by stops 6 and7, respectively securedly affixed to the surfaces 4 and 5 and keepingthe two surfaces 4 and 5 at a distance. The document US 5,750,420describes a method for fabricating such a structure, wherein the usefullayer 1 is kept at a distance from the substrate 3 by stops 6 and 7. Itcomprises partial etching of the sacrificial layer 2, leaving a spacerblock 8 with a width of about one micron, then partial etching of theuseful layer 1 so as to form the stops 6 and 7, and then etching toeliminate the spacer block 8. This method thus requires three etchingsteps, the first of which is difficult to master. The arrangement of thespacer block 8, and consequently of the stops, is determined by etchingfronts propagating from lateral orifices 11.

The article “The effect of release-etch processing on surfacemicrostructure stiction” by R. L. Alley et al. (Solid-State Sensor andActuator Workshop, 5^(th) Technical Digest, 22 June 1992, pages 202-207)describes a method enabling microstructures to be released by etchingand mentions that the surface roughness enables the separation betweensurfaces to be increased. A suspended structure, initially attached by asacrificial layer to a highly doped substrate, is released by etching ofthe sacrificial layer, for example using hydrofluoric acid. Thesubstrate can be formed by an n-doped material or by a p-doped material,for example by a method using B₂O₂. The suspended structure comprises apolysilicon layer doped with nitrogen for one hour at 1050° C. Afteretching of the sacrificial layer, the n-doped polysilicon is muchrougher than an amorphous material used in a comparison test.

The document U.S. Pat. No. 6,004,832 describes a method for fabricatinga nitride layer suspended on a conducting substrate. The nitride layeris first deposited on an insulating layer that is at least partiallyetched. Then the surface of the substrate, formed by a highly dopedmaterial, is made rough by chemical means, for example using potassiumhydroxide (KOH).

OBJECT OF THE INVENTION

It is an object of the invention to remedy the shortcomings of knownmethods and, more particularly, to prevent sticking of the useful layerand of the substrate, while simplifying the fabrication method.

According to the invention, this object is achieved by the accompanyingclaims and, more particularly, by the fact that the method comprisesdeposition, before doping, of a mask on at least a predetermined part ofthe useful layer so as to delineate at least one doped zone and at leastone non-doped zone of said surface, one of said zones forming a stopafter the superficial etching phase.

It is also an object of the invention to achieve a component comprisinga suspended useful layer, attached by fixing means to a substrate,characterized in that it is obtained by a method according to theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

Other advantages and features will become more clearly apparent from thefollowing description of particular embodiments of the invention givenas non-restrictive examples only and represented in the accompanyingdrawings, in which:

FIGS. 1 and 2 represent a component according to the prior art,respectively before and after etching of the sacrificial layer.

FIGS. 3 and 4 represent doping steps of a particular embodiment of amethod according to the invention.

FIG. 5 represents an epitaxy step of a particular embodiment of a methodaccording the invention.

FIGS. 6 to 8 illustrate different etching steps of a particularembodiment of a method according the invention.

DESCRIPTION OF PARTICULAR EMBODIMENTS

In FIG. 3, the useful layer 1, made of silicon, is initially attached bythe sacrificial layer 2, made of silica, to the layer 3 forming thesilicon substrate. As represented by the arrows in FIG. 3, a first stepof a method for separating the useful layer 1 consists in doping thebottom surface 4 of the useful layer 1, arranged in contact with thesacrificial layer 2. Doping is performed through the useful layer 1.

The doped silicon surface has the property of etching more quickly thana non-doped silicon surface and, in addition, with a greater roughness.Thus, after complete etching of the sacrificial layer, a superficialetching phase of the surface 4 increases the roughness of the doped partof the surface (FIG. 8), which enables the adhesion forces between theopposite surfaces of the useful layer and of the layer forming thesubstrate to be reduced, and thus enables sticking of the useful layerand of the substrate to be prevented or at least limited.

In the embodiment represented in FIG. 3, a mask 9 is deposited beforedoping on a central part of the top face of the useful layer 1. Thus,the mask 9 delineates a non-doped zone of the bottom surface 4 of theuseful layer 1. As this non-doped zone etches more quickly than thedoped zones, it constitutes a stop 6 at the end of the process, afterthe superficial etching phase (FIG. 8).

In a second doping step of the method, represented in FIG. 4, the topsurface 5 of the substrate layer 3 can be partially doped. As before, amask 9 can delineate a non-doped central zone.

The doping steps are preferably performed by ion implantation, thedoping elements being taken from the group comprising Boron, Phosphorusand Arsenic. The energy of the ions determines the depth of penetrationin the material and thus enables the bottom surface 4 of the usefullayer 1 and the top surface 5 of the layer 3 forming the substrate to bedoped selectively. For example, a silicon surface intrinsically doped byboron (P type doping) and having a resistivity of 1 Ω.cm, is doped byboron by ion implantation with an energy of 45 keV and a dose of 5×10¹⁵atoms/cm² over a thickness of 0.3 μm, giving a resistivity of 1.5. 10⁻³Ω.cm for the 0.3 μm thickness of the bottom surface 4 of the usefullayer 1. Ion implantation of boron applied on the same type of silicon,through a useful layer 1 of silicon of 0.21 μm and a sacrificial layer 2of silica of 0.4 μm, is performed, for example, with an energy of 240keV and a dose of 2×10¹⁴ atoms/cm², giving a resistivity of 0.01 Ω.cmover a thickness of 0.3 μm of the top surface 5 of the layer 3 formingthe substrate.

The doping doses, energies and thicknesses can be adjusted to thethicknesses to be passed through, the required roughness, the requiredselectivity of etching of the doped silicon compared with the non-dopedsilicon and the thickness to be etched, which, on the other hand, dependon the etching solution used and on the etching time. The resistivity ofthe doped zones is typically 10 or 1,000 times greater than that of thenon-doped zones, but this ratio can be higher or lower depending on thetype of doping and the etching solutions used.

Moreover, too weak doping does not enable the required roughness to beobtained, whereas in the case of excessive doping, the material etchestoo quickly, and controlling the etching and roughness is thereby moredifficult. However, excessive doping can be used to completely eliminatethe doped part.

To improve the efficiency of the doping steps, the initial useful layer1 (FIGS. 3 and 4) is preferably thinner than the final useful layerrequired (FIGS. 5 to 8). After doping, the thickness of the useful layer1 can thus be increased by an epitaxy step, represented in FIG. 5,generally using the same material as that of the initial useful layer 1,i.e. typically silicon, but not necessarily the same type of doping. Theresistivity of the materials can be respectively determined by a wellcontrolled doping rate. The final thickness of the final useful layer 1is typically about 20 μm, the initial useful layer being able, forexample, to have a thickness of about 0.3 μm.

In FIG. 6, vertical apertures 10 are machined by etching in the usefullayer 1 to successively enable the etching solutions of the sacrificiallayer 2 and for superficial etching of the respective surfaces 4 and 5of the useful layer 1 and of the substrate layer 3 to pass. The geometryand arrangement of the apertures 10 enable the dimensions of thesuspended part of the useful layer to be defined. The useful layer 1 issuspended by fixing means that are not represented.

The sacrificial layer 2 is typically removed, as represented in FIG. 7,by etching with hydrofluoric acid-based solutions. Superficial etchingis typically performed by a potassium solution and, preferably, by anaqueous solution containing K₂Cr₂0₇ and HF, for example of the “Secco”type. The thickness of the etched superficial layer is typicallycomprised between a few nanometers and 1 micron.

As illustrated in FIG. 8 (on a very enlarged scale), the superficialetching phase of the bottom surface 4 of the useful layer 1 and of thetop surface 5 of the substrate layer increases the roughness of thedoped zones. The non-doped zones, if any, remain flat and are etchedless deeply than the doped zones. Thus, the non-doped zones form stops 6and 7 arranged facing one another, keeping the two opposite surfaces 4and 5 at a distance, which, in combination with the roughness of theopposite surfaces 4 and 5, enables the risk of sticking of the surfaces4 and 5 to be further limited.

Generally, the bottom surface 4 of the useful layer 1 and the topsurface 5 of the layer 3 forming the substrate intrinsically comprisedoping elements of a predetermined type, i.e. N type or P type doping.The doping represented in FIGS. 3 and 4 is performed by the same type ofdoping elements and the non-doped zones therefore form (FIG. 8) thestops 6 and 7 at the end of the method. In an alternative embodiment,doping can be performed by doping elements of opposite type. In thiscase, the etching rate is lower in the zones doped by the dopingelements of opposite type than in the non-doped zones, and the stops arethen formed by the doped zones, at the end of the method.

Whereas in the embodiment represented in FIG. 7, the whole of thesacrificial layer is removed after doping and before superficial etchingof the surfaces 4 and 5, in a particular embodiment, after doping andbefore the superficial etching phase, the sacrificial layer 2 is onlypartially etched, leaving at least one spacer block 8 between the layer3 forming the substrate and the useful layer 1, as represented in FIG.2. As in the document U.S. Pat. No. 5,750,420, the superficial etchingphase of the surfaces 4 and 5 then uses the spacer block 8 as a mask soas to form the stops 6 and 7 in the surfaces 4 and 5. At the same time,it increases the roughness of the doped free zones of the surfaces 4and/or 5. In addition, after the spacer block 8 has been removed, anadditional superficial etching phase of the surfaces 4 and 5 can beperformed to increase the roughness of the doped surface of the stops 6and/or 7. This enables formation of stops of larger size than the flatstops without increasing the risk of sticking. Control of partialetching of the sacrificial layer 2 forming the spacer block 8 is thusmade easier. Control of the dimensions of the spacer block is lesscritical than in the case described in the document U.S. Pat. No.5,750,420 as the surface is rough.

The method applies particularly to thin sacrificial layers 2 of SiO₂ thethickness whereof is comprised between a few tens of nanometers and afew microns and preferably about 400 nanometers. For example, substrates3 of the silicon on insulator (SOI) type are particularly suitable, inparticular substrates obtained by separation by implantation of oxygen(SIMOX) preferably having an oxide thickness of 400 nanometers, or dessubstrates of the Unibond® type obtained by the Smart-Cut® methodpreferably having an oxide thickness of 1 to 3 microns.

The invention is not limited to the particular embodiments represented.In particular, doping of one of the opposite surfaces 4 and 5 only maybe sufficient to prevent sticking of the surfaces. Doping of both of thesurfaces is useful in certain conditions of use, for example in the caseof high perpendicular accelerations or of large differences of potentialbetween the two surfaces, etc. . . .

Moreover, in the case of doping of the two opposite surfaces 4 and 5,one of the surfaces can be completely doped, whereas doping of the othersurface can be partial, for example by means of a mask 9. It is alsopossible to obtain a rough, substantially flat, surface facing at leastone stop arranged on the other surface. Such a component can beobtained, for example, by removing, the mask 9 after the first dopingstep. In a general manner, the different doping steps can be performedusing different masks. The stops 6 and 7 on the surfaces 4 and 5 can beof any number and arranged in any way.

In the prior art described in the document U.S. Pat. No. 5,750,420, thearrangement of the stop is determined by the arrangement of the spacerblock remaining after partial etching of the sacrificial layer, anarrangement determined by the etching fronts. Unlike this prior art, thearrangement of the stop of the method according to the invention iscontrolled by the arrangement of the mask deposited on the useful layer.It is thus possible, for example by forming the mask by lithography, tocontrol the arrangement and shape of the stops 6 and 7 very precisely.The mask 9 is delineated, for example, by photolithography, preferablyhaving a resolution of about 0.3 micrometers. Photolithography makes itpossible to delineate, with a good reproducibility, stops 6 and 7 ofvery precise lateral dimension, in the planes of the surfaces 4 and/or5, for example with a lateral dimension of 2 micrometers, plus or minus0.3 micrometers. The lateral dimension of the stops 6 and 7 defines thecontact zone between the opposite surfaces of the useful layer 1 and ofthe substrate and thus determines the contact force between the usefullayer 1 and the substrate. Control of the lateral dimension of the stops6 and 7 thus enables the contact forces to be controlled.

Furthermore, the height of the stops 6 and 7, perpendicularly to theplanes of the surfaces 4 and/or 5, does not significantly influence thecontact between the useful layer 1 and the stops 6 and 7. Thesuperficial etching time of the surfaces 4 and 5, which defines theheight of the stops without modifying the lateral dimensions of thestops, is therefore not critical. Due to the doping contrast, etching isin fact highly selective.

1-11. (canceled)
 12. Method for separating a useful layer, initiallyattached by a sacrificial layer to a layer forming a substrate, methodcomprising at least partial etching of the sacrificial layer, doping,before etching of the sacrificial layer, of at least a part of thesurface of at least one of the layers in contact with the sacrificiallayer and, after etching of the sacrificial layer, a superficial etchingphase of said surface so as to increase the roughness of the doped partof the surface, method comprising deposition, before doping, of a maskon at least a predetermined part of the useful layer so as to delineateat least one doped zone and at least one non-doped zone of said surface,one of said zones forming a stop after the superficial etching phase.13. Method according to claim 12, wherein, said surface intrinsicallycomprising doping elements of a predetermined type, doping is performedby doping elements of the same type, the stop being formed by thenon-doped zone.
 14. Method according to claim 12, wherein, said surfaceintrinsically comprising doping elements of a predetermined type, dopingis performed by doping elements of opposite type, the stop being formedby the doped zone.
 15. Method according to claim 12, wherein the mask isdelineated by photolithography.
 16. Method according to claim 15,wherein the photolithography has a resolution of about 0.3 micrometers.17. Method according to claim 12, comprising, after doping, an epitaxystep increasing the thickness of the useful layer.
 18. Method accordingto claim 12, wherein doping is performed by ion implantation, the dopingelements being taken from the group comprising Boron, Phosphorus andArsenic.
 19. Method according to claim 12, wherein superficial etchingis performed by an aqueous solution containing K₂Cr₂0₇ and HF. 20.Method according to claim 12, wherein the sacrificial layer iscompletely etched before the superficial etching phase of said surface.21. Method according to claim 12, comprising after doping and before thesuperficial etching phase of said surface, partial etching of thesacrificial layer so as to leave at least one spacer block between thelayer forming the substrate and the useful layer, the superficialetching phase of said surface using the spacer block as mask, so as toform at least one stop in said surface, removal of said spacer block, anadditional superficial etching phase of said surface so as to increasethe roughness of the surface of the stop.
 22. Component comprising asuspended useful layer, attached by fixing means to a substrate,obtained by a method according to claim 12.